How to extrude an irregular shape into a cylinder?
Multiple Methods
Extrude and To Sphere

Extrude your edge loop upwards.
Type Shift + Alt + S &gt; 1 to execute the To Sphere command.
If you have overlapping vertices, try using the smooth (W &gt; O) a couple of times and then the To Sphere command again.


LoopTools Circle

Extrude your edge loop upwards and select it.
Choose the Circle of the LoopTools addon. It comes with Blender and has to be activated first in the User Preferences &gt; Addons. It is located in the Specials menu. W &gt; LoopTools &gt; Circle.

Bridge to Circle

Select your edge loop and note the vertice-count in the info bar.
Add a circle, set its number of vertices equal to the edge loops.
Position the circle and select both, circle and previous edge loop.
Do the bridge edge loops command. Ctrl + E &gt; L